1. Field of the Invention
The present invention relates to an image forming apparatus, such as photocopier, facsimile, printer, plotter, or multifunctional machine incorporating several of those imaging functions, and more particularly, to an electrophotographic image forming apparatus employing a guide assembly in a media conveyance path along which a recording medium travels after processing through a fixing device that permanently fixes a toner image in place on the recording medium with heat and pressure.
2. Discussion of the Background
In electrophotographic image forming apparatus, such as photocopiers, facsimiles, printers, plotters, or multifunctional machines incorporating several of those imaging functions, an image is formed by attracting toner particles to a photoconductive surface for subsequent transfer to a recording medium such as a sheet of paper. After transfer, the recording sheet is forwarded to a post-transfer sheet conveyance path, where it undergoes a fixing process to permanently fix the toner image in place on the recording sheet by melting and settling toner with heat and pressure.
Various types of fixing processes are known in the art, most of which employ a pair of parallel, elongated fixing members, one being a fuser member heated for fusing toner and the other being a pressure member pressed against the heated one, which together define a heated area of contact, called a fixing nip, through which the recording sheet is ultimately passed under heat and pressure for fixing the toner image. Such fixing members include paired cylindrical rollers, each with or without an endless belt looped for rotation around the roller circumference, rotating in pressure contact with each other to drive a recording sheet forward through the fixing nip.
Currently, an increasing number of electrophotographic systems employ a fixing device with an enlarged fixing nip for obtaining a high-speed, high-productivity, and compact-size fixing process. Enlarging or elongating the fixing nip along the sheets conveyance path means an increased period of dwell time during which a recording sheet is subjected to heat and pressure within the fixing nip, allowing for intensely heating and pressing the recording sheet at higher processing speeds without requiring large equipment or large amounts of energy consumed for heat generation.
Typically, a long, large fixing nip is created by using a combination of a primary fixing roller and an additional, secondary fixing member or roller with a fixing belt entrained around the combined fixing rollers to form one side of the fixing nip (i.e., either a heated, “fuser” side, or a pressed “pressure” side), opposite to which is a single roller that forms the other side of the fixing nip. In this arrangement, the fixing nip extends over a relatively large area of contact, including where the primary fixing roller contacts the opposite roller as well as where the secondary fixing roller contacts the opposite roller, larger than that obtained with a single fuser roller and a single pressure roller.
One problem encountered with use of an enlarged fixing nip is the difficulty in properly conveying a recording sheet from the fixing nip downstream to between a pair of conveyance rollers, such as those employed to forward the recording sheet into an output unit for eventual delivery outside the apparatus, or those defining a heated, glossing nip therebetween which further processes the recording sheet with heat and pressure to provide a glossy and smooth appearance to the printed image after fixing.
The difficulty arises where a recording sheet processed under intense heat and pressure through the fixing nip develops a curl or bend with its edge pointing away from the proper conveyance path to interfere with adjacent surfaces upon exiting the fixing nip. If not corrected, such deformation or deviation causes the sheet to jam the conveyance path, or otherwise crease upon entering the downstream conveyance roller pair. This problem is particularly pronounced where the conveyance rollers are disposed immediately downstream from the fixing nip as is the case with the glossing rollers, which are ideally disposed as close as possible to the fixing nip to provide fixing and glossing processes with high thermal efficiency.
Various methods have been proposed to provide proper sheet conveyance downstream of the fixing nip in the image forming apparatus. A typical approach is to employ a sheet guide that defines a guide surface extending along the sheet conveyance path to push against the edge or surface of a curled recording sheet for correcting deformation and deviation during passage from the fixing nip to downstream processes.
For example, one conventional method uses a guide assembly formed of a plate combined with a protruding structure, such as rollers, spurs, or ribs, that defines an uneven, non-planar guide surface to pass a recording sheet in an intended direction along the sheet conveyance path. According to this method, using the protruding tracking structure minimizes the area of contact between the guide surface and the recording sheet, which is intended to prevent print defects caused by rubbing the printed face of the recording sheet against the guide surface.
Unfortunately, the conventional guide assembly does not work well where the recording sheet has a significant curl or bend developed through intense heat and pressure, as is the case with processing through the extended fixing nip. Rather, instead of correcting deformation and deviation, this method can adversely affect the print quality where the recording sheet smudges, jams, or creases by striking against the uneven guide surface of the protruding structure.